1. Field of the Invention
The present invention relates to a vehicle communication system for performing data communication between ECUs mounted on a vehicle.
2. Description of the Related Art
Along with a recent increase in electronic instrumentation mounted on a vehicle, a number of ECUs (electronic control units) have been provided in a vehicle. Further, in order for ECUs to perform data communication to each other, communication lines for data transmission are cabled between ECUs, and junction connectors (hereinafter, referred to as “J/Cs”) for branching the communication lines are provided.
FIG. 1 is an explanatory view showing a construction of a conventional vehicle communication system. As shown in the drawing, in the vehicle communication system, a J/C 103 is connected to an ECU 101 serving as a starting point, and a communication line branches in two lines through a J/C 103. The branch lines are connected to J/C 104 and J/C 105, respectively.
Six ECUs 102 are connected to the J/C 104 and J/C 105, respectively, and the junction points within the J/C 104 and J/C 105 are grounded through termination resistors RT, respectively.
Further, the length of the communication line connecting the ECU 101 serving as a starting point and the J/C 103 is set at 4 m, and the lengths of the communication lines connecting the J/C 103 and the J/C 104, and the J/C 103 and the J/C 105 are both set at 4.5 m. Lengths of the communication lines between the J/C 104 and each ECU 102, and between the J/C 105 and each ECU 102 are set at 2 m. In the drawing, Tx represents transmission and Rx represents reception.
In the vehicle communication system with the above construction, once a signal is transmitted from the ECU 101 as the starting point to each ECU 102 when an ON state (dominant) is turned into an OFF state (recessive) so-called ringing in which a signal waveform largely oscillates in the vertical direction sometimes occurs.
FIGS. 2 and 3 are characteristic views showing signal waveforms at different parts of the vehicle communication system when a signal is transmitted from the ECU 101. In FIG. 2, a curve S101 represents a voltage generated at the output terminal of the ECU 101, and a curve S102 represents a voltage generated at the J/C 104. A curve S103 shown in FIG. 3 represents a voltage generated at the input terminal of the ECU 102.
In FIGS. 2 and 3, an output of a transmission signal is switched from the OFF state to the ON state at time t101. Such an output signal from the ECU 101 then reaches a predetermined voltage level and a desired signal is transmitted to the other ECU 102.
At time t102, the ON state of an output of a transmission signal is switched to the OFF state. The output of a transmission signal is then turned off, but the waveform S101 at the output terminal of the ECU 101 shows large vertical oscillations due to ringing, and it continues to oscillate until time t103, the next turn-on time.
Therefore, as shown in the curve S102 in FIG. 2 and the curve S103 in FIG. 3, voltages generated at the J/C 104 and the input terminals of the ECUs 102 on the reception side, continue to have waveforms vertically oscillating with small amplitudes. This is attributed to the fact that the junction points within the J/C 104 become “sections” of a standing wave generated between the ECU 101 and the J/C 104. Once such ringing occurs, an error occurs in a communication signal between the ECU 101 and the ECUs 102, causing a problem of degraded communication accuracy.
Furthermore, since ringing occurs because of the junction points within the J/C 104 becoming “sections” of a standing wave as stated above, ringing is not something that always occurs. Characteristic impedance changes depending on various conditions including the length of communication lines between ECUs and J/Cs, the size of the termination resistors RT connected to the J/C 104 and J/C 105, the number of ECUs 102 connected to the J/C 104 and J/C 105. A large scale of ringing or no ringing may occur depending on the magnitude of the characteristic impedance.
Therefore, it is possible to prevent ringing by determining positions of the ECUs 101 and 102 and the J/Cs 103 to 105, lengths of communication lines and the like in the vehicle based upon rough prediction of conditions with which no ringing may occur. Nevertheless, in reality, characteristic impedance may change due to variation in lengths of communication lines in different vehicles on which the ECUs, J/Cs, and the communication lines are mounted, and addition of ECUs, which may result in ringing.
A proposed method for preventing ringing without fail is disclosed, for example, in Japanese Patent Laid-Open Publication No. 2000-151153. In this method, drivers are connected to respective bus lines to drive the bus lines. Thus, characteristic impedance is prevented from changing.